1. Field of the Invention
The present invention relates to a physical quantity sensing device with a bridge circuit and more particularly relates to a sensing device that includes a pair of sensor elements, the impedance of which changes with a physical quantity to be measured. As used herein, the “physical quantity” is any quantity that can be measured with sensor elements, and refers to any of a broad variety of quantities such as forces (various forces including torque load), current, voltage, light quantity, and temperature.
2. Description of the Related Art
Magnetostrictive load sensing devices, including magnetostrictive sensor elements, have been developed for many years. A magnetostrictive sensor element is an element made of a magnetostrictive material, the initial permeability of which changes with the given load, and senses a variation in the initial permeability of the magnetostrictive material as a variation in the impedance (e.g., inductance and resistance) of a sensing coil, for example. Examples of preferred magnetostrictive materials include magnetic materials, soft magnetic materials and ultramagnetic materials such as ferrous alloys, iron-chromium based alloys, iron-nickel based alloys, iron-cobalt based alloys, pure iron, iron-silicon based alloys, iron-aluminum based alloys, and permalloys.
FIG. 1A is an equivalent circuit diagram showing a typical sensing circuit in a conventional magnetostrictive load sensing device. The bridge circuit shown in FIG. 1A has first and second input points N1 and N2, to which an AC voltage is supplied, and first and second output points S1 and S2, which are connected to a differential amplifier (not shown). The AC voltage is supplied to the first and second input points N1 and N2 from an AC voltage generator 10.
In the bridge circuit shown in FIG. 1A, magnetostrictive sensor elements SE1 and SE2 are connected in parallel to each other. A bridge circuit of this type will be referred to herein as a “parallel bridge circuit”. Load sensing devices with such a parallel bridge circuit are described in Japanese Patent Application Laid-Open Publications Nos. 5-60627, 10-261128 and 2001-356059 and Japanese Utility Model Application Laid-Open Publication No. 5-45537, for example.
The sensitivity of a magnetostrictive sensor element changes due to a temperature variation and therefore requires some temperature compensation. In the prior art, such temperature compensation is performed via a temperature sensor and a sensitivity variation corrector including thermistors (see Japanese Patent Application Laid-Open Publication No. 2001-356059, for example).
Japanese Patent Publication No. 2776693 discloses a method of performing temperature compensation on the sensing characteristic of a sensing device by measuring the DC resistance of an AC signal detecting coil. This method is advantageous because no special temperature sensing elements are needed anymore and because the temperature of a coil with a temperature coefficient can be measured directly.
However, according to the method disclosed in Japanese Patent Application Laid-Open Publication No. 2001-356059, not only the thermistors as temperature sensing elements but also the temperature sensor and sensitivity corrector are needed, thus requiring an overly complicated circuit configuration. Then, the circuit board including those circuits and elements should have an increased area and the manufacturing cost would increase significantly, too.
Likewise, the method disclosed in Japanese Patent Publication No. 2776693 needs a means for measuring the DC resistance separately, thus requiring a more complicated circuit configuration, a bigger circuit board and a higher manufacturing cost also.